The present invention relates to a circuit for a piezoelectric positioning or control element or the like which is alternately charged and discharged. The positioning element has a plurality of piezoelectric bodies designed to be charged and discharged in reverse phase.
In piezoelectric positioning elements as are known, for example in the disclosure of the German patent application P 30 40 563.1 incorporated herein by reference, piezoelectric components are provided whose electrically effected elongation or contraction is employed for mechanical work output. As a rule, such positioning elements have a long piezoelectric body, which, for example consists of a stack of discs placed atop one another or of long strips secured to one another. The discs or the strips are piezo-ceramic material such as, for example, lead zirconate titanate, varium titanate and the like.
Such a positioning element represents an electrical capacitance to be charged for an electrical voltage applied thereto. During the course of this charging operation, the corresponding piezo-ceramic body executes an elongation or contraction, depending upon the operational sign of the piezoelectric effect or, respectively, of the electro-mechanical coupling factor which is effective in the corresponding case. In an analogous manner, the contraction or enlargment effect can also be exploited which occurs when a previously electrically charged piezo-ceramic body is short-circuited and discharges its electrical capacitance.
Although the changes in length of a piezo-ceramic body are relatively slight upon application of electrical voltage, perhaps comparable to the movement of a tappet which is driven by a cam, a piezo-ceramic positioning or control element is in a position to exert rather high forces and to produce a corresponding work output. The electrical energy which must be exerted for this purpose, however, is not only this work output but, rather, the reactive energy component of the charging of the electrical capacitance added thereto, said component being a multiple of the mechanical power output. For the sake of completeness, let it also be pointed out that a high voltage having a magnitude of from one thousand to several thousand volts is required for the operation of a piezoelectric positioning element as specified. This voltage has to be controlled in accordance with the desired positioning operation, and a corresponding electronic circuit has to be made available for generating and controlling said voltage. In particular, the overall energy requirement plays a part in the positioning operation when the positioning element is to be driven from a battery or when only a low voltage DC of, for example, 12 volts, is available.